Part CBasic
Rule 21: Definitions
Defines each navigation light by color, arc and position so aspect can be interpreted correctly.
Detailed Explanation
Each navigation light has a precisely defined colour, arc, and sector so that aspect and status can be correctly interpreted.
A
Masthead light — white, 225°, from right ahead to 22.5° abaft beam each side.
B
Sidelights — green starboard/red port, each 112.5°; under 20 m may be combined in one lantern on centerline.
C
Sternlight — white, 135°, 67.5° from right aft each side.
D
Towing light — yellow, same characteristics as sternlight.
E
All-round light — 360°.
F
Flashing light — 120+ flashes per minute.
Key Points
- Masthead: 225° white, forward half
- Sidelight arcs: 112.5° each (green starboard, red port)
- Sternlight: 135° white, abaft
- Towing light: yellow, same arc as sternlight
Examples
- You see a white light with a 225° arc above the vessel's fore-and-aft line. This is a masthead light as defined by Rule 21(a), visible from dead ahead to 22.5° abaft each beam.
- A vessel shows a yellow light visible over a 135° arc astern. Rule 21(c) defines this as a towing light — it has the same sector as a sternlight but is yellow instead of white.
Common Mistakes
- Confusing the arc of a sternlight (135 degrees) with that of a masthead light (225 degrees).
- Forgetting that a towing light is yellow, not white, and covers the same 135-degree stern arc.
- Miscalculating sidelight arcs as 90 degrees instead of the correct 112.5 degrees each.